An unremarkable and slow-growing plant has stunned scientists after they found
it had the world's largest genome – 50 times bigger than that of our own
species.

It is an unremarkable and rather fragile plant, but a flowering herb that frustrates gardeners for being notoriously difficult to grow has stunned scientists after they discovered it has the world's largest genome.

The DNA contained within Paris japonica dwarves all other plant and animal genomes that have been analysed so far. It is 50 times longer than the human genome, even though our species is thought to be one of the most complex and advanced on the planet.

A genome - the biological code which directs how every organism develops - is made up of the DNA found inside almost all living cells.

The slow-growing plant, which is native to the mountains of the Japanese island of Honshu but is also found in gardens in the UK, boasts more than 150 billion base pairs – the basic building block that links together to form DNA – in its genome. Humans have just three billion base pairs.

If stretched out, the genetic information contained within just one cell of Paris japonica would stretch more than 328 feet – taller than Big Ben – while the genome from a human cell would stretch just 6.5 feet.

The discovery has added to questions about why our own genome is so comparatively small. It may also lead to the discovery of new types of drugs.

Dr Ilia Leitch, a geneticist at Kew Gardens' Jodrell Laboratory who was part of the team that made the discovery, which is published in the Botanical Journal of the Linnean Society, said: "When we started looking at the plant on our machine to measure genome size it was clear there was something odd about Paris japonica.

"When we worked out just how big it was, I was staggered. I had to keep checking to be sure. It is amazing how the cells pack all of that DNA in there.

"Other members of the family that Paris japonica is part of have medicinal qualities so there could well be some that we can identify from the genome.

"Genome size does not necessarily relate to the complexity of an organisms. We can now start probing what is going on in these really big genomes and understand what is going on.

"It can tell us what restricts where plants can grow and how they are going to respond to climate change. While many factors affect how and where plants grow, it now seems that genome size is one of the factors."

The smallest genome found so far belongs to a bacterium called Carsonella ruddii, which has fewer than 160,000 base pairs. The smallest plant genome belongs to a carnivorous plant called Genlisea margaretae with 63.4 million base pairs, which if stretched out would measure just 1.6 inches.

The previous record holder for the largest genome was the marbled catfish, which had 130 billion base pairs and would stretch 288 feet.

Dr Leitch said that having such a large genome could help explain why Paris japonica was so slow-growing and vulnerable to pollution and other extreme conditions.

She added: "Having that much DNA does have consequences for the plant – plants with big genomes are at greater risk of extinction, more sensitive to pollution.

"Having so much DNA means every time you want to divide a cell in order to grow, you have to replicate all that DNA and it takes a lot longer."

Listen to a podcast by the Natural Environment Research Council's online journal Planet Earth about the giant genome.